The objective of this proposal is to elucidate the mechanisms by which hormones regulate gene expression in the developing rat mammary gland and to investigate how these regulatory mechanisms may have deviated in hormone-dependent breast cancer. Two rat mammary cancer model systems will be studied: the DMBA- and NMU-induced mammary carcinomas. Casein synthesis will be utilized as a specific biochemical marker for the expression of differentiated function in the mammary gland. Procedures have been developed for measuring both the rates of synthesis and turnover of casein messenger RNA and the rate of casein synthesis in mammary tissue under various hormonal manipulations, thus permitting an evaluation of hormone action at the transcriptional, post-transcriptional and translational levels. Our studies will initially be performed in vivo and then extended to in vitro systems. Prolactin induction of casein mRNA in organ culture will be used as a unique system for understanding the mechanism by which peptide hormones regulate specific gene expression. The multiple interaction of several other hormones, including progesterone and hydrocortisone, which modulate the response to prolactin, will also be studied. Using cloned DNAs derived from the three individual casein mRNAs the arrangement of the casein genes, the nature of their primary transcription products and the mechanism of the coordinate induction of the casein proteins will be investigated. In addition, the transcription of casein mRNA will be studied in isolated nuclei in order to identify any possible factors mediating the peptide hormone induction of a specific gene product. A careful analysis of unique sequence DNA transcription and nuclear RNA processing will be performed in both the normal mammary gland and the hormone-dependent mammary carcinomas using the latest techniques of molecular hybridization. Both homologous and cross-hybridization experiments will be utilized to characterize those unique RNA transcripts which are present in mammary cancer. Significant advances in our understanding of mammary cancer and hormone action in general will result from these studies of the regulation of gene expression.